The present invention relates to a power cable coating composition capable of affording an insulation coating superior in heat resistance.
Various plastic materials have heretofore been used as power cable insulating materials. Particularly, olefinic polymers are superior in various characteristics, including electrical and mechanical characteristics and chemical stability. Above all, low density polyethylenes prepared by a high pressure process radical polymerization are inexpensive, exhibit a reduced dielectric loss, are superior in processability, can be greatly improved in heat resistance by crosslinking, and are less likely to exhibit a tree phenomenon induced by incorporation of a foreign matter such as a residual catalyst as compared with polymers prepared by ionic polymerization. Because of these various advantages, olefinic polymers are in wide use as insulating materials for electric wires and power cables.
Most important characteristics required for such insulation coating for power cable include heat resistance at service temperature (usually up to about 90.degree. C.) and that at elevated temperatures higher than the melting point, for example, at the time of short-circuiting. The method most commonly adopted for improving such heat resistance is to increase the degree of crosslinking (gel percentage) of the insulation coating layer, and to this end it is desirable to use a highly crosslinkable resin.
Particularly, coupled with the recent labor-saving tendency, improvement of productivity and increase of voltage and capacity have been demanded for power cable, and at the same time it has been desired to improve the performance, e.g. heat resistance, of the insulation coating.
In conventional power cable insulating coatings comprising low density polyethylenes, the degree of crosslinking (gel percentage) cannot be fully improved in usual amounts of peroxides used because of insufficient crosslinkability of the low density polyethylenes, and the resultant coatings are inferior in heat resistance, that is, the percentage heat-deformation at elevated temperatures above melting points remarkably increases. Besides, an attempt to increase the molding speed for improving the productivity results in lowering of the gel percentage because of an insufficient crosslinking speed, and eventually the heat resistance is deteriorated.
On the other hand, if the amount of peroxide used is increased in order to improve the gel percentage and thereby attain a satisfactory heat resistance, scorch is apt to occur during extrusion coating, thus not only making a long-time continuous run impossible, but also causing deterioration of electrical characteristics of the insulation coating on power cable.
Further, the use of ethylene copolymer rubber alone or in combination with low density polyethylenes has also been proposed as a coating material in characteristics such as the resistance to dielectric deterioration as compared with a single use of polyethylene, but the heat resistance of crosslinked polyethylene is deteriorated by the incorporation of ethylene copolymer rubber. And in the use of ethylene copolymer rubber alone, deterioration results not only in heat resistance but also in various other characteristics such as resistance to oil, to ozone and to wear.